Motion of Orbiting Satellites

AP Physics 1· difficulty 4/5

A satellite orbits a planet of mass MM in a circular orbit of radius rr (measured from the planet's center). What are the orbital speed vv and orbital period TT?

  • A

    v=GM/rv = \sqrt{GM/r}, T=2πr3/(GM)T = 2\pi\sqrt{r^3/(GM)}

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  • B

    v=GM/rv = GM/r, T=2πr/GMT = 2\pi r/\sqrt{GM}

  • C

    v=2GM/rv = \sqrt{2GM/r}, T=2πr2/(GM)T = 2\pi r^2/(GM)

  • D

    v=GMrv = \sqrt{GM r}, T=2πr/vT = 2\pi r/v

Explanation

Setting gravity equal to centripetal force: GMm/r2=mv2/rGM m/r^2 = m v^2/r, so v=GM/rv = \sqrt{GM/r}. Then T=2πr/v=2πr3/(GM)T = 2\pi r/v = 2\pi\sqrt{r^3/(GM)} (Kepler's third law).

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